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United States Patent |
5,232,676
|
Wolff
,   et al.
|
August 3, 1993
|
Process for the biological purification of waste air streams
Abstract
In a process for the biological elimination of halogen- and
sulfur-containing gaseous pollutants from a waste air stream, the waste
air stream is passed through an intermittently moistened trickling filter
tower. In a preferred variant of the process, individual sections of the
surface of the trickling filter are subjected to intermittent
phase-displaced moistening. Cyclic moistening of the trickling filter
sections can be achieved in this way. The volume-time yield in the
biological purification of gas streams can be considerably improved by
this process.
Inventors:
|
Wolff; Felix (Cologne, DE);
Melin; Thomas (Cologne, DE)
|
Assignee:
|
Bayer Aktiengesellschaft (Leverkusen, DE)
|
Appl. No.:
|
941769 |
Filed:
|
September 4, 1992 |
Foreign Application Priority Data
Current U.S. Class: |
423/210; 95/210; 210/615; 423/DIG.17 |
Intern'l Class: |
B01D 053/00 |
Field of Search: |
55/90,97
210/615
423/DIG. 17,210
|
References Cited
U.S. Patent Documents
3665677 | May., 1972 | Koch | 55/56.
|
4385988 | May., 1983 | Hypponen | 210/150.
|
4443337 | Apr., 1984 | Otani et al. | 210/602.
|
4662900 | May., 1987 | Ottengraf | 55/90.
|
4869824 | Sep., 1989 | Melin et al. | 210/615.
|
5085766 | Feb., 1992 | Born | 210/150.
|
5089137 | Feb., 1992 | McKown | 210/615.
|
5116506 | May., 1992 | Williamson et al. | 210/610.
|
5123936 | Jun., 1992 | Stone et al. | 55/8.
|
Foreign Patent Documents |
3641442 | Jun., 1988 | DE.
| |
Primary Examiner: Straub; Gary P.
Assistant Examiner: Vanoy; Timothy C.
Attorney, Agent or Firm: Sprung Horn Kramer & Woods
Parent Case Text
This application is a continuation of application Ser. No. 07/740,275,
filed Aug. 5, 1991, now abandoned.
Claims
We claim:
1. A process for the biological elimination of halogen or sulfur-containing
gaseous pollutants from a waste air stream, comprising: passing the waste
air stream through a moistened trickling filter, and intermittently
moistening the trickling filter by repeatedly applying a liquid for
moistening for a moistening period and following each moistening period
with a non-moistening period when no liquid is applied for moistening.
2. A process as claimed in claim 1, wherein the trickling filter is
intermittently moistened by spraying with a liquid.
3. A process as claimed in claim 1, wherein the intermittent moistening is
carried out in cycles and the frequency of the moistening cycles is
regulated to maintain a constant pH value of the solution flowing off from
the trickling filter.
4. A process as claimed in claim 1, wherein the intermittent moistening is
carried out in cycles and the frequency of the moistening cycles is
regulated in dependence upon the content of carbon in any form in the gas
flowing off the trickling filter.
5. A process as claimed in claim 1, wherein the trickling filter has a
plurality of segments and the plurality of segments of the trickling
filter are subjected to intermittent time phase-displaced moistening.
6. A process as claimed in claim 4, further comprising subjecting several
trickling filters in tandem or parallel to intermittent time
phase-displaced moistening.
7. A process according to claim 2, wherein the spraying is for 20 seconds
to 3 minutes from 2 to 10 times per hour.
Description
BACKGROUND OF THE INVENTION
This invention relates to a biological process for the purification of
waste air streams laden with sparingly water-soluble substances of which
the biological degradation is accompanied by the formation of acids.
The biowasher process is known from the literature. In this process, the
organic impurities are washed out from the waste air stream in an absorber
and the washing liquid is biologically purified in an activated sludge
tank. The purified liquid is then recirculated into the absorber and
re-charged. However, the uptake capacity of the liquid for the substances
mentioned above is so small that the recirculation stream between the two
units becomes so large that the process cannot be used for economic
reasons.
U.S. Pat. No. 0 249 861 A2 describes a process in which the absorber and
the activated sludge reactor are integrated into a a single apparatus, a
plate column. Compared with the spray or jet washer, the scope of
application is distinctly broader. However, since the necessary number of
stages and hence the pressure loss decrease with decreasing solubility of
the waste air ingredients in water, there are also limits to this process.
In another biowasher process (DE 3 641 442 A1), silicone oil is added to
the washing liquid so that the liquid is able to absorb more organic
substances. However, removal of the salts formed is problematical. In
addition, silicone oil is always discharged with the salt and has to be
replaced.
The biofilter process is unsuitable for the elimination of substances of
which the degradation is accompanied by the formation of HCl or H.sub.2
SO.sub.4 because the biofilm acidifies so that the degradation activity of
the microorganisms comes to a standstill.
The trickling filter process is mentioned in VDI Richtlinie 3478. In
contrast to the biowasher process, the microorganisms are immobilized in
the trickling filter process. The gas is passed through a packing in which
the mass transfer of the organic constituents and the oxygen takes place.
In addition, a liquid stream is passed over the packing, neutralizing the
H.sup.+ ions, removing the salt accumulating and supplying the bacteria
with nutrient salts. Through the immobilization of the microorganisms,
salts can be removed without any loss of bacteria. Hitherto, trickling
filters have mainly been used for odor elimination.
Neutralization of the acids formed during the degradation of A and Cl
compounds is difficult on account of the limited pH range in which
microorganisms are viable and the uneven flow through packings. If too
little liquid is distributed through the packing, the biofilm acidifies
and the degradation activity of the organisms is reduced. However, if the
sprinkling density is too high, the liquid preferentially blocks channels
in which there are large quantities of biomass. These organisms then take
no further part in elimination of the waste air ingredients so that
degradation performance deteriorates.
SUMMARY OF THE INVENTION
The problem addressed by the present invention was to ensure that the pH
value in the biofilm would not fall excessively and the liquid stream
would not block the channels of the trickling filter.
According to the invention, the solution to this problem is characterized
in that the trickling filter is intermittently moistened. The trickling
filter is preferably sprayed with a liquid for 20 s to 3 mins. 2 to 10
times per hour. A buffered nutrient salt solution is advantageously used
as the liquid.
In another embodiment of the process according to the invention, the
frequency of the moistening cycles is regulated in dependence upon the pH
value of the solution flowing off from the trickling filter. In this case,
therefore, an optimal moistening frequency is established in dependence
upon the pH value.
In one advantageous embodiment, the process according to the invention is
carried out by intermittent phase-displaced moistening of various parts of
a trickling filter.
In another embodiment, several trickling filter units subjected to
intermittent phase-displaced moistening may be arranged in tandem or
parallel.
The process according to the invention affords the following advantages:
In the elimination of halogen- or sulfur-containing gaseous pollutants from
a waste air stream, the volume-time yield can be increased by a factor of
three to four compared with known processes. Accordingly, with large
volumetric gas flows, as for example in room extraction systems, the
saving of time and costs is considerable.
The additional investment costs compared with conventional biological waste
air purification are relatively low. The process according to the
invention can also be carried out in existing plants with relatively
little outlay on additional equipment.
The phase-displaced moistening of the partial surfaces has the advantage
that the deterioration in elimination performance in the part being
sprayed does not significantly affect the elimination performance of the
plant as a whole.
Embodiments of the invention are described in detail in the following with
reference to the accompanying drawings, wherein:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a trickling filter tower for carrying out the process
according to the invention.
FIG. 2 is a flow chart of the process with its regulated moistening cycle.
FIG. 3 is a cross-section through a trickling filter with circularly
sprayed partial sections.
FIG. 4 is a diagram explaining the phase-displaced intermittent moistening
of the trickling filter sections shown in FIG. 3.
FIG. 5 shows a trickling filter tower with units arranged in parallel.
DETAILED DESCRIPTION OF THE INVENTION
In the trickling filter tower 1 shown in FIG. 1, the waste gas 2 to be
purified is introduced at the upper end and distributed uniformly over the
cross-section by a gas distributor 3. A perforated plate 4 is arranged
beneath the gas distributor 3 for further uniform distribution. The key
component of the trickling filter tower 1 is the trickling filter 5 which
consists of crosslinked polyurethane foam having a specific surface of 500
m.sup.2 /m.sup.3. The microorganisms required for purification of the
waste air are immobilized on the foam. The trickling filter 5 is moistened
by a series of spray nozzles 6 arranged in parallel (spray cones 7). The
liquid then trickles through the trickling filter 5 and collects in the
sump 8 at the bottom of the trickling filter tower 1. The liquid in the
sump 8 is recirculated to the spray nozzles 6 by a pump 9. The purified
gas issues through the pipe 10. Fresh liquid or a nutrient salt solution
can be introduced through the connection 11 on the liquid circuit.
The pump 9 does not operate continuously, but rather intermittently under
the power of a timed motor. The duration and frequency of the moistening
cycles, i.e. the spray cycles, depends upon the nature and degree of the
gas pollution. Experience has shown that good separation results are
obtained when the trickling filter is sprayed with liquid for 20 s to 3
mins. (cycle time) 1 to 5 times per hour (frequency).
With substances of which the biological degradation is accompanied by the
formation of acid, the time between two spraying cycles and hence the
spraying frequency is determined by the degree of acidification. With
other substances, for example toluene or acetone, the trickling filter has
to be sprayed when the microorganisms are no longer sufficiently supplied
with nutrients.
The following substance-specific optimal spraying cycles were empirically
determined:
______________________________________
Concentration
Substance
[mg/m.sup.3 ] Cycle time
Frequency
______________________________________
Methylene
<400 2 Mins. 3 Times/h
chloride
Methylene
<400 2 Mins. 4 Times/h
chloride
Toluene <1,000 30 s. 2 Times/h
" >1,000 30 s. 2 Times/h
Acetone and
toluene
______________________________________
With chlorinated hydrocarbons or substances of which the degradation is
accompanied by the release of H.sup.+ ions or OH.sup.- ions, it is of
advantage to regulate the moistening frequency through the pH value. A
corresponding flow chart is shown in FIG. 2. A nutrient salt solution
buffered with sodium hydroxide is used as the spray liquid. The nutrient
solution is stored in a tank 11 while the sodium hydroxide is stored in a
tank 12. The nutrient solution is fed to the column sump by means of the
pump 13. The measurement signal of a pH measuring station on the sump of
the column is fed to a controller 17. The controller 17 controls the pump
14 so that a constant pH value prevails in the sump of the column.
As already explained with reference to FIG. 1, the circulation pump 9 pumps
the liquid from the sump 8 to the spray nozzles 6. Another pH measuring
station is present at the outlet 15 of the trickling filter. The pH signal
is delivered to a controller 16 which controls the pump in such a way that
a new spraying cycle is initiated when the pH value of the liquid running
down from the trickling filter 5 falls below a predetermined value. In
this case, the control system ensures that the moistening frequency and,
optionally, the cycle time are automatically adjusted.
Irrespective of the described control through the pH value, the frequency
of the spray cycles can also be controlled through the values for the
total carbon content of the clean gas (10) issuing from the plant. The
total carbon content can be measured by a flame ionization detector (FID).
In this method of control, the spraying frequency is best increased when
the elimination performance (difference between input and output) falls
below a predetermined value.
A further development of the process is described in the following with
reference to FIGS. 3 and 4. The moistened circular areas 17a to
17g--associated with the spray cones 7 of the spray nozzles 6--on the
surface of the trickling filter are shown diagrammatically in FIG. 3. The
surfaces 17a to 17g are not sprayed at the same time, but instead with
phase displacement. The intermittent phase-displaced moistening is
illustrated in FIG. 4. The seven different partial diagrams are associated
with sections 17a to 17g on the surface of the trickling filter. The time
is plotted on the abscissa while the volumetric flow of liquid through the
spray nozzles 6 associated with the circular areas 17a to 17g is plotted
on the ordinate. The rectangular pulses 18 each correspond to a spray
cycle. As shown in FIG. 4, the phase displacement in the moistening of the
surface 17b relative to the surface 17a is .DELTA.T. The surface 17c is
moistened at a time phase-displaced by 2.DELTA.T relative to the surface
17a. . . by 3.DELTA.T in the case of the surface 17d, etc. The sections of
the trickling filter are thus circularly (cyclically) moistened.
Another embodiment of a trickling filter tower which has been successfully
used in the process according to the invention is shown in FIG. 5. In this
case, the trickling filter does not consist of a single block, but of
several blocks 5a, 5b, 5c and 5d which are arranged in parallel. The waste
air stream to be purified is divided among the individual sections 5a to
5d by means of the tube distributor 19. The purified partial streams
issuing at the lower end are then combined in the collecting pipe 20 and
removed. In this embodiment, too, the individual sections of the trickling
filter can be subjected to phase-displaced moistening. To this end, the
trickling filter sections 5a to 5d are provided with separate liquid
inlets and outlets. A common sump 8 is present at the bottom of the
trickling filter tower. The two embodiments shown in FIG. 5 and FIG. 3 may
also be combined with one another so that intermittent phase-displaced
moistening takes place both in the horizontal direction and in the
vertical direction.
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